(1) Field of the Invention
The present invention relates to an antenna structure and a television receiver having the antenna structure. More specifically, it relates to a technique to electrically change directivity of the antenna structure.
(2) Description of the Related Art
Methods for adjusting receiving conditions on antennas include a mechanical adjustment method where the antennas are tilted and rotated, and an electrical adjustment method where directivity of the antennas is altered electrically without moving the antennas. Examples of the antennas employing the electrical adjustment method are such as ESPAR antennas (Patent Reference: Japanese Laid-Open Patent Application No. 2002-118414) and diversity antennas in which one of antenna elements having different directivity is selected so as to change the directivity of the diversity antenna.
Such antennas include dipole elements or monopole elements. These elements are required to be disposed either with a sufficient distance from a metal casing and a circuit substrate of a transmission apparatus, or standing on the metal casing or circuit substrate as an earth plate. This makes it inconvenient to carry about the transmission apparatus, because the dipole elements or monopole elements extend outward from the transmission apparatus.
Even if the dipole elements and monopole elements are disposed parallel to the casing of the transmission apparatus (the earth plate), bulkiness of the antenna remains, because an interval between two elements should be at least λ/4. Especially when the transmission apparatus is a thin television receiver, the bulkiness of the antenna ruins its portability and appearance. Because the television broadcast radiowaves are horizontally polarized, the dipole elements and monopole elements have to be disposed on a top of the casing of the transmission apparatus in order to be disposed parallel to the casing of the transmission apparatus (the earth plate).
On the other hand, inverted F antennas and loop antennas have been known as low profile antennas (Non-Patent Reference: Naohisa Goto, “Illustrated Text: Antenna” The Institute of Electronics, Information and Communication Engineering, 1995, pp. 225–227). It is considered that the above problem of bulkiness can be solved by employing these low profile antennas.
However, a technique to change the directivity of the inverted F antennas and loop antennas has not been established.